Tom Stevenson and his task force of strategically placed experts provide readers with the latest inside information from every wine region in Wine Report 2009. Read a selection of Dr Ronald Jackson’s contribution on Wine Science.

Oxygen and Cork

The relative merits of cork versus alternative closures will probably remain unsettled for decades. However, the debate has had a beneficial effect on research into bottle closures. The latest in a slew of recent studies has investigated how oxygen enters wine in stoppered bottles and the dynamics of the process. Lopes and co-workers have discovered that most of the oxygen that enters wine closed with natural cork does so during the first month
(Oxygen Ingress into Wine Bottles).

Subsequently, ingress falls sharply, becoming negligible after a year. By comparison, technical cork (composed of cork particles) is far superior in limiting oxygen uptake, whereas synthetic cork is considerably worse.

Why does oxygen ingress vary so much over time with natural cork? Scientists cannot as yet give a definitive answer, but a major factor probably involves the oxygen contained within the cellular structure of cork. Cork consists of multiple layers of empty cells that act like miniature gas pockets. A cork of average length consists of a series of cells about 500 cells deep. When the stopper is compressed for insertion into the bottle, pressure on the air in each cell increases. This either diffuses outward into the surrounding air or is absorbed by the wine. Because air contains about 23 per cent oxygen, this can contribute several milliliters of oxygen to the wine. In contrast technical cork is made from cork particles and glue compressed together in a mould. That means that some of the oxygen in the cork cells is forced out during production. In addition, technical cork is less resilient than natural cork and is compressed less prior to insertion into the bottle. The greater oxygen permeability of artificial (synthetic) corks is undoubtedly explained by the slow but continual ingress of oxygen through the stopper.

Only with time, experience, and controlled study will the benefit/deficit consequences of limited oxygen ingress on wine development become clear. There is no perfect bottle closure, just options with different consequences – some good, some bad, some indifferent. In the past, we experienced only the consequences associated with quality variations in natural cork. The wine world is much more complex today, with closure options now encompassing not only natural cork, but also hybrid cork, technical cork, a diversity of synthetic corks, glass stoppers, crown caps, and screwcaps. As the famous oenologist Vernon Singleton said years ago, “Wine is, and must remain, I feel, one of the few products with almost unlimited diversity…keeping the consumer forever intrigued, amused, pleased, and never bored.”

Primary TCA Source

Since its discovery in the early 1980s, the mouldy off-odour of TCA has been a major concern of the wine industry. Considerable success has been achieved in removing TCA from commercial cork supplies, and the use of synthetic corks and screwcaps prevents its occurrence. Nevertheless there is still the nagging question about the origin of TCA. Without this knowledge, measures to eliminate it will remain curative rather than preventive.

Simpson and Sefton conclude from their review that the primary source of contamination occurs in the forests where cork oak is grown. The most likely culprit is PCP, a biocide used extensively since the 1930s. One of the techniques microbes use to detoxify PCP involves adding methyl groups to the compound. The unfortunate by-product is the generation of TCA, with its intense mouldy odour at infinitesimally small concentrations.

Since the current use of PCP is much reduced, if not eliminated, and the harvest cycle for cork removal is normally nine years, maybe we can hope that in some 10 to 20 years’ time the incidence of TCA off-odour will diminish, even if cork is not treated to remove the compound.

Off-odours

When discussing off-odours, it is as if one were discussing incorrect grammar. The difference is that, whereas accepted grammar texts exist, there are no equivalents for wine off-odours. What is considered an off-odour often depends on its vinous context. I have seen renowned Sauternes, spoiled by the nail-polish-remover odour of ehyl acetate, lauded for its quality, and prestigious Rhône wines praised for their wonderful terroir when spoilt by the manure smell of ethylphenols. Even sherry is so oxidized (in a complex way) that it is considered one of its principal quality attributes. If experts have problems separating off-odours from contextual issues, what hope does the average consumer have? Years ago, before “corked” became the safe expression to use when rejecting wine, “vinegary” was commonly used – that is, unless you wanted to be more sophisticated, in which case “oxidized” was preferable. This is intended not to poke fun at experts or consumers but to highlight the legitimate problem of defining what a wine fault is, to whom, and under what conditions.

A valuable study would be for returned wine to be assessed by qualified tasters as well as chemically analyzed. It would be useful to know what correlation exists between the stated reason for rejection and any fault detected. My suspicion is that there would be little correlation. If so, this is a lamentable situation. How can winemakers improve their efforts to produce faultless wines if what the clientele objects to cannot be correctly identified?

Dr Ronald Jackson in Wine Report 2009
Selection of text and choice of illustrations by Aad van der Werf

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